Don't crash when swiping up if there are no TaskViews

This can happen in LockTask mode, after enabling Overview. In that case,
you are locked to the current task, but are allowed to go to overview
but no tasks are shown there.

Bug: 139917483
Change-Id: I2180cc5e72c343977ea53cc02907ed8dac0f8bc6
Merged-In: I2180cc5e72c343977ea53cc02907ed8dac0f8bc6